The present invention relates to a cathode-ray tube for a projector, the heat dissipation characteristic of which is improved.
In the cathode-ray tube for a projector, a phosphor screen formed on the inner surface of a faceplate is scanned with an electron beam from an electron gun. An image of an effective surface is thus obtained. The image of the effective surface is focused on a projection screen by an optical system arranged outside or inside the tube. The cathode-ray tube for a video projector must be activated at a final anode potential, that is, a potential of +25 kV to +30 kV on the phosphor screen. The phosphor screen of the cathode-ray tube for a video projector must be scanned with an electron beam in which the current density is a multiple of ten time the density of the electron beam used for a cathode-ray tube for television. 90% of the input power of 0.05 to 1.0 W/cm.sup.2 will vanish as heat loss. The heat loss causes the temperature of the phosphor to rise in the effective area which is scanned with the electron beam and in the temperature of the outer surface of the faceplate corresponding to the effective area. Therefore, the following problems are presented.
(1) Luminous efficiency is degraded owing to the rise of temperature of the phosphor.
(2) The faceplate may be cracked by a stress which is caused by a difference between the temperature rise on the outer surface corresponding to the effective area and the temperature rise on the peripheral outer surface which surrounds the outer surface corresponding to the effective area.
In order to solve the first problem, a cooling mechanism on the inner surface of the faceplate may be adopted. However, such a cooling mechanism is not proposed in practice. In fact, a cooling mechanism which cools the phosphor indirectly from the outer surface of the faceplate is currently used. Further, in order to solve the second problem, it is very effective to reduce the temperature difference between the outer surface corresponding to the effective area and the peripheral outer surface by conducting heat on the outer surface of the faceplate corresponding to the effective area to the peripheral outer surface and the peripheral part of the faceplate. For this purpose, conventionally, various structures have been proposed. However, these cooling devices are large in size, resulting in low relative luminance. Thus, the satisfactory cathode-ray tube for a projector cannot be obtained.